This invention relates to an enclosure through which hot gases are passed, and in particular, to such an enclosure for use in furnace sections of once-through vapor generators.
In general the furnace enclosure of a once-through vapor generator is made up of a plurality of walls defined by upright tubes arranged for the through-flow of a vaporizable fluid such as water. It is known to connect the wall forming tubes to each other by metal fins, thereby providing an all-welded gas-tight furnace enclosure.
In some furnace enclosure arrangements the wall tubes are grouped into several individual "passes" which are interconnected for serial flow of fluid. When the passes are disposed around the enclosure in a side-by-side manner, the fin connecting the edge tube of one pass to the adjacent edge tube of another pass can be subjected to high shear stresses, since the passes and tubes thereof experience different thermal expansion as a result of the absorption of heat from the hot gases passing through the furnace. Therefore, it has been proposed to eliminate the connecting fin between adjacent tubes of different passes, thereby spacing the passes apart and allowing the passes to move longitudinally relative to each other. It has also been proposed to connect a skin casing or seal plate to the adjacent passes in order to make the furnace enclosure gas-tight. A variety of seal plate designs have been proposed for use with such an arrangement. Some seal plates consist of metal sheets formed of a plurality of rhomboid shaped parallelogram areas, others consist of plates formed of a plurality of cup-shaped elements, and still others consist of a trough-shaped member having corrugations formed therein. Each of these seal plate arrangements is intended to compensate for the differential expansion which could occur between the adjacent passes of the enclosure.
However, an enclosure defined by spaced apart passes and provided with a skin casing or seal plate for gas-tightness could succumb to a problem known as "stack effect" whereby hot gasses passing through the enclosure flow into the space existing between the enclosure wall forming tubes and the skin casing or seal plate. The hot gases can rise up through that space, and heat the casing or seal plate, thereby causing it to fail. It has been proposed to provide overlapping fins between the adjacent tubes of different passes, in order to protect the seal plate from the direct in-flow of heated gases, but such a design would not be gas-tight at the overlap and hence would not preclude stack effect if hot gases did leak into the space between the seal plate and the wall.
In yet another enclosure design it has been proposed to use a slip joint consisting of three fins stuffed with asbestos rope and covered by skin casing. This arrangement would also not preclude stack effect, if leakage occured.
The instant invention provides a substantially gas-tight enclosure through which hot gases are passed, including a seal plate which compensates for differential expansion between adjacent passes of the enclosure, and means for filling the space between the wall and the seal plate which prevent overheating of the seal plate.